1. Field of the Invention
The present invention relates to an inner wall of a nacelle of a turbomachine such as an airplane turboprop or turbojet.
2. Description of the Related Art
In general, any nacelle inner wall comprises an outer annular fan casing that surrounds the fan blades of the turbomachine, and that is suitable for having fastened therein an annular layer of abradable material surrounding the fan blades. At its downstream end, the fan casing has an annular flange for fastening to an intermediate casing of the turbomachine, and at its upstream end, it has an annular flange for fastening to the downstream end of a ring having its upstream end connected to a transverse annular partition for connecting together the inner and outer walls of the nacelle. In particular, the transverse partition may serve to stiffen the nacelle and also to provide a flame-arrestor function.
A soundproofing inner annular structure is arranged upstream from the fan blades to reduce the propagation of sound waves generated by the blades while in operation. The soundproofing structure is situated upstream from the abradable layer of the fan casing and it extends axially therefrom.
In the prior art, in order to install a one-piece soundproofing structure, the structure is fastened in the vicinity of its downstream end to the above-mentioned ring, and at its upstream end to the air intake lip of the nacelle.
Proposals have already been made to form the annular structure by assembling together a plurality of structural acoustic panels. Nevertheless, the connecting zones between the panels constitute zones in which there is a sudden change of acoustic impedance, thereby significantly degrading the soundproofing performance of the structure. The soundproofing structure is thus preferably made as a single substantially cylindrical panel that extends axially from the abradable layer of the fan casing to the air intake lip of the nacelle.
Nevertheless, that technology presents several drawbacks. For maintenance purposes, it is necessary to remove the cylindrical panel in order to remove the air intake lip, and that takes a relatively long time. Furthermore, the panel is structural in order to transmit forces between the air intake lip and the intermediate casing, and as a result it is relatively heavy. It is also expensive and complex to make. Finally, the flanges connecting the fan casing to the ring are situated at a short axial distance from the fan blades and they may be exposed to violent impacts and to high levels of mechanical stress in the event of a fan blade being lost.